1. Field of the Invention
The present invention relates to a method of annealing a semiconductor film using a laser light (hereinafter referred to as laser annealing) and to a laser irradiation apparatus for conducting the method (apparatus including a laser and an optical system for guiding a laser light emitted from the laser to an object to be processed). Also, the present invention relates to a method of manufacturing a semiconductor device which includes a step of irradiation of the laser light. Note that the term semiconductor device mentioned here indicates semiconductor devices in general which can function by utilizing semiconductor characteristics, and includes electro-optical devices such as a liquid crystal display device and a light emitting device and an electronic device including the electro-optical device as a component.
2. Description of the Related Art
In recent years, there has been widely studied a technique of conducting laser annealing to a semiconductor film formed on an insulating substrate made of glass or the like to crystallize the film, thereby improving crystallinity. Silicon is often used for the semiconductor film. In this specification, a method of crystallizing a semiconductor film with a laser light to obtain a crystalline semiconductor film is referred to as laser crystallization.
A glass substrate has such advantages that: it is inexpensive and has a wealth of processability; and a large-area substrate can be easily manufactured from the glass substrate in comparison with a synthetic quartz glass substrate often used in the prior art. This is the reason why the above study is being made. Further, the reason a laser is used for crystallization from choice residues in that the glass substrate has a low melting point. The laser can impart high energy only to a semiconductor film without increasing a substrate temperature much. Further, crystallization can be performed with the laser in a short time. Thus, remarkably high throughput is provided with the laser in comparison with a heating means using an electrically-heated oven.
Since a crystalline semiconductor is constituted of a large number of crystal grains, it is also called a polycrystalline semiconductor film. The crystalline semiconductor film formed by performing laser annealing has high mobility, and thus is actively used for an active matrix liquid crystal display device, for example, which is manufactured by forming thin film transistors (TFTs) using the crystalline semiconductor film and forming TFTs for a pixel portion and for a driver circuit on, for example, a glass substrate.
For example, crystallization of a semiconductor film can be performed by using a typical laser such as an excimer laser or a YAG laser. However, the YAG laser emits a coherent light having extremely high coherence. Therefore, in the case where the YAG laser is used for laser crystallization, it is difficult to form a laser light having a uniform energy distribution on an irradiation surface or in the vicinity thereof. It can be considered that the cause of the extremely high coherence of the YAG laser is that the YAG laser has a coherent length of about 10 mm or longer while the excimer laser has a coherent length of several to several tens of xcexcm.
Further, in case of using the YAG laser, it is desirable that the laser light emitted from the laser is converted into a second harmonic using a non-linear optical element and that the converted light is formed into the laser light having a rectangular shape or elliptical shape on the irradiation surface by using an optical system. However, the non-linear optical element used for converting the laser light into the harmonic is penetrated with the laser light, and thus is required to have sufficient heat-resistance and durability. As the laser has a larger output, the deterioration of the non-linear optical element becomes larger.
The present invention has been made in view of the above, and an object of the present invention is therefore to provide a laser irradiation method in which an energy distribution of a laser light becomes uniform on an irradiation surface even with the use of an excimer laser or a YAG laser having high coherence and a laser irradiation apparatus for conducting the method. Further, another object of the present invention is to reduce deterioration of a non-linear optical element in the case where a harmonic of a laser light having energy corresponding to that of a high-output laser light is generated. Still another object of the present invention is to provide a method of manufacturing a semiconductor device including a semiconductor film obtained by performing crystallization of a semiconductor film and activation of an impurity element in accordance with the above-described laser irradiation apparatus and laser irradiation method.
According to a structure of the present invention disclosed in this specification, there is provided a laser irradiation apparatus, characterized by including: a plurality of lasers; means for controlling oscillation of the plurality of lasers; means for synthesizing a plurality of laser lights emitted from the plurality of lasers into a laser light; means for condensing the laser light on an irradiation surface or in the vicinity thereof; and means for moving the laser light at least in one direction.
The above structure of the present invention is characterized in that the laser is one or plural kinds of lasers selected from the group consisting of a solid laser, an excimer laser, an Ar laser, and a Kr laser of continuous oscillation or pulse oscillation type. As the solid laser, there may be given a YAG laser, a YVO4 laser, a YLF laser, a YAlO3 laser, a glass laser, a ruby laser, an alexandrite laser, a Ti:sapphire laser, and the like.
The above structure of the present invention is characterized in that at least one laser light among the plurality of laser lights is converted into a harmonic by a non-linear optical element. The laser light is converted into the harmonic before the plurality of laser lights are synthesized into a laser light, whereby deterioration of the non-linear optical element is reduced, which leads to reduction in cost.
The above structure of the present invention is characterized in that the means for synthesizing a plurality of laser lights emitted from the plurality of lasers into a laser light has a waveguide or a thin film polarizer. Thus, there is formed a laser light having an energy density corresponding to that of a laser light emitted from a large-output laser.
The above structure of the present invention is characterized in that the means for moving the laser light has a galvanometer or polygon mirror and an fxcex8 lens. By using the above components, the laser light can be moved on the irradiation surface.
The above structure of the present invention is characterized in that when the irradiation surface is provided slant to the laser light, and when the laser light has a beam width W and a substrate has a thickness d, an incident angle xcfx86 to the irradiation surface of the laser light satisfies the equation: xcfx86xe2x89xa7arcsin (W/2d). If the laser light is made incident at the incident angle xcfx86, the reflection light caused by a surface of the substrate and the reflection light caused by a back surface of the substrate do not interfere with each other. Thus, uniform laser light irradiation can be performed. The irradiation surface is provided slant to the laser light as described above, whereby there can be prevented the occurrence of the interference in case of using the YAG laser having a long coherent length.
Further, according to a structure of the present invention disclosed in this specification, there is provided a laser irradiation method, characterized by including: synthesizing a plurality of laser lights into a laser light; condensing the laser light on an irradiation surface or in the vicinity thereof; and irradiating the laser light while moving the laser light.
The above structure of the present invention is characterized in that the laser is one or plural kinds of lasers selected from the group consisting of a solid laser, an excimer laser, an Ar laser, and a Kr laser of continuous oscillation or pulse oscillation type. As the solid laser, there may be given a YAG laser, a YVO4 laser, a YLF laser, a YAlO3 laser, a glass laser, a ruby laser, an alexandrite laser, a Ti:sapphire laser, and the like.
The above structure of the present invention is characterized in that at least one laser light among the plurality of laser lights is converted into a harmonic by a non-linear optical element. The laser light is converted into the harmonic before the plurality of laser lights are synthesized into a laser light, whereby deterioration of the non-linear optical element is reduced, which leads to reduction in cost.
The above structure of the present invention is characterized in that the means for synthesizing a plurality of laser lights emitted from the plurality of lasers into a laser light has a waveguide or a thin film polarizer. Thus, there is formed a laser light having an energy density corresponding to that of a laser light emitted from a large-output laser.
The above structure of the present invention is characterized in that the means for moving the laser light has a galvanometer or polygon mirror and an fxcex8 lens. By using the above components, the laser light can be moved on the irradiation surface.
The above structure of the present invention is characterized in that when the irradiation surface is provided slant to the laser light, and when the laser light has a beam width W and a substrate has a thickness d, an incident angle xcfx86 to the irradiation surface of the laser light satisfies the equation: xcfx86xe2x89xa7arcsin (W/2d). If the laser light is made incident at the incident angle xcfx86, the reflection light caused by a surface of the substrate and the reflection light caused by a back surface of the substrate do not interfere with each other. Thus, uniform laser light irradiation can be performed.
Further, according to a structure of the present invention disclosed in this specification, there is provided a method of manufacturing a semiconductor device, characterized by including: synthesizing a plurality of laser lights into a laser light; condensing the laser light on an irradiation surface or in the vicinity thereof; and irradiating the laser light to a semiconductor film while moving the laser light, thereby performing crystallization.
Further, according to a structure of the present invention disclosed in this specification, there is provided a method of manufacturing a semiconductor device, characterized by including: introducing an impurity element to a semiconductor film; synthesizing a plurality of laser lights into a laser light; condensing the laser light on an irradiation surface or in the vicinity thereof; and irradiating the laser light to the semiconductor film while moving the laser light, thereby performing activation of the impurity element or recovery of the crystallinity of the semiconductor film.
The above structure of the present invention is characterized in that the laser is one or plural kinds of lasers selected from the group consisting of a solid laser, an excimer laser, an Ar laser, and a Kr laser of continuous oscillation or pulse oscillation type. As the solid laser, there may be given a YAG laser, a YVO4 laser, a YLF laser, a YAlO3 laser, a glass laser, a ruby laser, an alexandrite laser, a Ti:sapphire laser, and the like.
The above structure of the present invention is characterized in that at least one laser light among the plurality of laser lights is converted into a harmonic by a non-linear optical element. The laser light is converted into the harmonic before the plurality of laser lights are synthesized into a laser light, whereby deterioration of the non-linear optical element is reduced, which leads to reduction in cost.
The above structure of the present invention is characterized in that the means for synthesizing a plurality of laser lights emitted from the plurality of lasers into a laser light has a waveguide or a thin film polarizer. Thus, there is formed a laser light having an energy density corresponding to that of a laser light emitted from a large-output laser.
The above structure of the present invention is characterized in that the means for moving the laser light has a galvanometer or polygon mirror and an fxcex8 lens. By using the above components, the laser light can be moved on the irradiation surface.
The above structure of the present invention is characterized in that when the irradiation surface is provided slant to the laser light, and when the laser light has a beam width W and a substrate has a thickness d, an incident angle xcfx86 to the irradiation surface of the laser light satisfies the equation: xcfx86xe2x89xa7arcsin (W/2d). If the laser light is made incident at the incident angle xcfx86, the reflection light caused by a surface of the substrate and the reflection light caused by a back surface of the substrate do not interfere with each other. Thus, uniform laser light irradiation can be performed. Thus, the crystallization of the semiconductor film, or the activation of the impurity element and the recovery of the crystallinity of the semiconductor film can be performed.